uss: uss_procs.c format-overflow warning

uss: uss_procs.c format-overflow warning

GCC 9 introduced new warnings/errors and is flagging a sprintf with a
format-overflow warning.  With --checking-enabled, this error is causing
uss_procs.c to fail during compile.

A file name with the full path is being composed and the size of the
buffer was triggering a possible format-overflow warning/error.

Use asprintf to allocate the buffer dynamically instead of using a
buffer sitting on the stack (reducing the stack requirements by 2K).

Produces new error message if asprintf returns an error.

Reviewed-on: https://gerrit.openafs.org/13664
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 41ee558329560bce037ad2860282d8b49aa11b2d)

Change-Id: I5c5866142ae17c92017201fb567f847b5c2907a0
Reviewed-on: https://gerrit.openafs.org/13729
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>